Pre-stretched plastic films



Sept. 3, 1 6 OLE-BENDT RASMUSSEN 3,

PRE'STRETCHED PLASTIC FILMS Filed July 6, 1964 lllllllllllllll P 11 FIG. 3

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INVENTOR. OLE'BENDT RASMUSSEN A TTORNEVS United States Patent 01 lice3,400,041 PRE-STRETCHED PLASTIC FILMS Ole-Bendt Rasmussen, Copenhagen,Denmark, assignor to Phillips Petroleum Company, a corporation ofDelaware Continuation-impart of application Ser. No. 215,743,

Aug. 8, 1962, now Patent No. 3,257,488. This application July 6,1964,Ser. No. 380,599

Claims priority, application Denmark, Nov. 19, 1963,

5,412/ 63 3 Claims. (Cl. 161117) This invention relates to a partiallyoriented film which can be stretched further in a very uniform manner.In another aspect it relates to an initial cold-stretching of a film ofsuch material followed by a final stretching for the purpose ofuniformly orienting the material.

This application is a continuation-in-part of my copending US.application Ser. No. 215,743, filed Aug. 8, 1962, now Patent No.3,257,488.

The term film as used here should be understood as including films,bands, sheets, lay-flat tubing and like forms of material.

Cold-stretching, which means stretching at temperatures substantiallybelow the melting range of the material to be stretched, is desirable inthat it imparts greater toughness and tear resistance to the materialthan does a stretching near the melting range.

Considerable difficulties are met when films of high densitypolyethylene or ethylene copolymers or isotactic polypropylene arecold-stretched, said difirculties being due to the high crystallinityand consequently the great stiffness of the material. Owing to this,great force is necessary to tear the crystalline elements out of theiroriginal connections and to enable orientation to set in. On the otherhand, once the original connections are broken, the flow of the chain.molecules speeds up without much hindrance, and is, therefore, difiicultto keep under control. When the process gets out of control, the generaleffect is that the orientation is concentrated in a very narrow zone orzones, and therefore the heat developed by the stretching cannot be ledaway quickly enough. The result is an instant and irregular temperatureincrease leading to rupture, probably because the material simply meltsat some places. Cooling, as for example, with a direct stream of water,is not sufficient, because the heat conductivity of the polymer is solow that only a small part of the stretching heat reaches the surfacesbefore rupture occurs.

Thus, if the film is cold-stretched without special precautions,orienting will set in at a stretching ratio of 1.25:1 or even less, butin an irregular manner, starting at randomly distributed places over thewhole area of the film to which the stretching forces are applied. Oncestretching has started at a locality in the film, it will continue forsome time, in and around that neighborhood, owing to the above explainedmechanism of the stretching process with the result that isolatedirregularly shaped spots or areas of strongly oriented material areformed surrounded by material which has not been oriented at all. Sinceby stretching in one direction, the film will tend to contract in thetransverse direction, it is easily understood that a rupture is likelyto occur, if some of the oriented areas grow so big as to unite withneighboring areas, since the contraction forces will generally be atopposite directions at the place of contact between two such areasjoining one another.

It has been found that with narrow strips of the film, stretching andcontracting of such strips take place in a narrow zone across the widthof the strip, the neckingdown zone. If the width is sufficiently small,for example, about 10 mm., ruptures can practically be avoided duringthe stretching. By cold-stretching at low velocities, e.g.,

3,400,041 Patented Sept. 3, 1968 in the ratio of 1.25:1, it was foundthat a network of shearing lines were formed in the material, i.e.,oriented areas in a regular criss-cross pattern of short lines formingan angle of about 4560 to the stretching direction, the orientation ofthe material, however, following the direction of stretching. By furtherslow stretching, preferably applying cooling with water, each of theshearing lines would gradually grow, till the film strip finally becametotally oriented.

The shearing lines were interpreted as microscopic necking-down zonesformed parallel to the direction of the maximum shear tension. Thelatter forms an angle of 45 to the direction of drawing, provided thatthe drawing forces are uniaxially directed.

In the cold-stretching of broader films, it was found that asubstantially uniform stretching over the whole width could be obtained,according to the present invention, by subjecting the film to an initialstretching in evenly spaced longitudinal zones, said initial stretchingbeing produced by advancing the film under tension over a series ofprojections, as between the grooves in a grooved bar or between theflutes in a cross-fluted roller, the tension on the film being adjustedso as to be of sulficient strength to start stretching the film at thecrown portions between the grooves or flutes, but not in the interveningparts. It was found that in this manner the initial orienting processcould be controlled so as to form a substantially regular pattern ofshearing lines or microscopic necking-down zones across the whole widthof the film.

An object of this invention is to provide films of crystalline polymerswhich can be cold stretched evenly without rupturing. Other objects,advantages, and features of this invention will be apparent to thoseskilled in the art from the following discussion and drawings in which:

FIGURE 1 is a schematic drawing of the cold-stretching process;

FIGURE 2 is a sectional view of a grooved bar;

FIGURE 3 is a detail of another type of grooved bar; and

FIGURE 4 is a plan view of a plastic sheet which has been partiallyoriented by drawing over the grooved bar.

The present invention can be used to orientfilms of any thermoplasticpolymeric material which is capable of being oriented by stretching attemperatures below the melting point of the polymer. Most frequentlysuch materials are highly crystalline polymers such as high densitypolymers of ethylene including polyethylene and copolymers of ethylene,with higher mono-l-olefins such as propylene or l-butene, andcrystalline or isotactic polypropylene and similar copolymers ofpropylene with ethylene or l-butene. These represent a well-known classof highly crystalline polymers of mono-l-olefins having 2 to 4 carbonatoms per molecule. Examples of the suitable polymers are polyvinylidenechloride, polyamides, polyethylene terephthalate, and the like. I preferto work with the polymers of mono-l-olefins which have a crystallinityof at least 80 percent as determined by nuclear magnetic resonance on aspecimen of polymer which is in thermal equilibrium. This can beachieved by heating the specimen to its melting point and then coolingit slowly to room temperature at a rate not exceeding 10 Fahrenheitdegrees per minute.

As indicated above, once the original connections between the moleculesof the material has been loosened, flow in the material becomes mucheasier, thus facilitating the subsequent stretching procedure forproducing strong orienting. Owing to the uniform pattern of shearinglines across the whole width of the film, each shearing line forms astarting place for the flow of material in the further stretching, andthe evolution of heat during the stretching is evenly distributed overthe whole area which is under the influence of the stretching forces,and probably to some extent reduced, with the result that the stretchingproceeds smoothly without ruptures.

In the above-mentioned copending application, Ser. No. 215,743, nowPatent No. 3,257,488, reference has been made to the use of across-fluted roller for producing a slipping line pattern, consisting ofshort slipping lines at an angle of 45 to the stretching direction in aprocess of orienting a film of an orientable polymer material.

In said copending application it is pointed out that the deficiencies ofknown orientation processes can be remedied by not leaving it to chancewhere the stretching process is initiated but by applying the stretchingforce in such a manner as to make the orientation progress in an orderlyfashion. This is effected by displacing locally applied pressures, whichare sufficient for producing a permanent stretching of the part of thesheet acted upon, crosswise over the breadth of the sheet in a narrowlateral zone as the sheet is being moved forward, and causing the saidlocally applied pressures to follow closely upon one another. The sheetcan then be stretched further in a known manner.

In this way it is possible to insure that the stretching takes placeover the entire breadth of the sheet in a lateral zone irrespective ofpossible variations in thickness and other irregularities in the sheet.By causing these lateral zones to follow closely upon one another insuccession or in a continuous manner, the orientation is obtained overthe entire length of the sheet. The method, therefore, has specialadvantages when applied to sheets which vary in thickness and in whichthis varying thickness tends to become accentuated by the stretchingprocess. Such sheets are frequently formed by the known blown-tubingprocess in which film is formed by inflation of an extruded tube, suchproducts being more or less laterally oriented by the molding process.The orientation process of this invention can also be applied quite wellto thick sheets which are difiicult to cold stretch.

As explained in the above-mentioned copending application, it isadvantageous to use a cross-fluted roller in the stretching apparatus.This fluted roller serves to produce suitable points of departure forthe orientation. At the passage of the sheet over the fluted roller aslight pleating is produced and at the same time the flutes cause aslight stretching to take place as manifested by the formation in thefilm of a slipping line pattern made up of short slipping lines at anangle of about 45 degrees to the longitudinal direction of the sheetwhich is also the direction of travel. The individual slipping linesthus formed provide the points for initiation of further orientationwhen the film is stretched longitudinally and the slipping line patternultimately disappears. It has been found that a sheet stretched in thismanner is particularly suitable for fiber production since it yieldsextremely fine and uniform fibers. Now the cross-fluted roller has beenimproved upon by the use of the grooved bar as described herein, makinguse of the same principle of initiating a pattern of slipping lines in aregular manner at closely spaced intervals across the breadth of asheet. The grooved bar has been found more suitable for this purpose,one of the points of difference from the fluted roller being the flatcrown portions of the ridges which separate the grooves.

The stretching ratio to be used in the initial stretching can be variedonly to a limited degree, and is preferably kept within the range from1.1 :1 to 1.4: 1. Below the prior value, i.e. increase of the length,the initial stretching is generally too feeble to form the shearinglines, which should be starting points in the subsequent stretching toobtain strong orienting, and at the upper limit for the drawing ratio,the microscopic necking-down tends to become inhomogenous. The bestresults are obtained by rising a stretching ratio in the initialstretching step of about 1.25:1, the pattern of shearing lines beingexceed ingly fine and of uniform distribution when using this ratio.

Even if great care is taken in producing the raw material for extrusionof the films of high density polyethylene or isotactic propylene used inthe present method, slight variations will inevitably occur from lot tolot of said material.

Thus it was observed that some lots gave a coarse pattern of shearinglines, and others a very fine one, and further it was noted that thelatter were best suited for the cold-stretching procedure.

Based upon the above described work, it was concluded that the startingpoints for the formation of shearing lines are inhomogenous places inthe material, such as, for example, accumulations of small molecules,bearing in mind that the molecular size varies, or the presence offoreign substances in the otherwise homogenous material. By deliberatelyintroducing a foreign substance into the main plastic before extrusionof the material, and it was found possible to regulate the fineness ofthe shearing line pattern.

According to an embodiment of the present method, therefore, a foreignmaterial is incorporated into the film material. Particularly goodresults have been obtained by admixing polyisobutylene orpolyvinylisobutyl ether, preferably in amounts of 5-15 by weight of thefilm material.

Referring now to the drawing which is described in connection with aspecific embodiment, in FIGURE 1, a film 5 of the material to bestretched, for example a 1 meter wide film of high density polyethyleneof specific weight 0.96, and melting index 0.2, into which has beenadmixed 10 weight percent of polyisobutylene, passes over rollers 6 and7 and a grooved bar 8 mounted close to said rollers 6 and 7 and then onto a pair of rollers 9 and 10.

The two pairs of rollers are driven at different peripheral speed so asto stretch the film at a stretching ratio of 1.25 1, the film velocitybeing 0.5 m./sec., and the stretching temperature being 20 C.

The grooved bar 8 has preferably a width or diameter of not more thanabout 12 mm., the surface which contacts the film being semicircular.

As it appears from FIGURE 2, the grooves are rounded, in this instanceof semicircular cross section, and are spaced apart, the interveningprojections having substantially flat tops. The width of the projectionsis preferably about one-third to one-half the width of the grooves.

FIGURE 3 illustrates an alternate design for the grooved bar which canbe turned from a rod. Although cylindrical, this bar is mounted so thatit does not rotate. Preferably there are about 1 to 2 grooves percentimeter of bar length and the radius of the bar is about 6 mm.

The film 5 contacts the curved upper surface of the grooved bar over anangle of about and is, owing to the tension, also in fairly closecontact with the bottom of the grooves over a slightly lesser angle. Asshown in FIG- URE 1, the portions of the film at the top of theprojections are under greater tension than the portions in the grooves.

As illustrated in FIGURE 4, the stretching starts at the top of theprojections, a fine pattern of shearing lines, which are parallel to theaxis of the grooved bar, being formed. On leaving the grooved bar, thefilm flattens out, and owing to the shift of tensioning forces, arougher crrss-cross of shearing lines at angles of 45 to the lines ofthe formerly mentioned fine pattern, is formed.

To fully develop the shearing line pattern, the film is passed twicemore through the stretching apparatus, and thus stretched to aboutdouble its length. Each stretching procedure reduces the width of thefilm by about 10%, and the thickness by about 15%.

By a subsequent stretching at room temperature, applying direct watercooling, and stretching to about 400% of the original length, theshearing line pattern disappears. The film has thus been equallystretched through and through, the resulting product being a stronglylongitudinally oriented film of great tear resistance in all otherdirections than that of the orientation. This product is valuable in themaking of laminates in which the directions of orientation of the plieslie at angles to each other, or in the manufacture of split fibers byfibrillating highly oriented film.

As will be apparent to those skilled in the art, various modificationscan be made in this invention without departing from the spirit or scopethereof.

I claim:

1. A pre-stretched film of orientable plastic material having regularlyspaced across the face thereof in longitudinal strips a pattern of finetransverse shearing lines and between said strips a criss-cross patternof shearing lines at angles of about 45 to 60 to said transverse lines,said film being uniformly orientable by simple cold stretching in thelongitudinal direction.

2. The film of claim 1 wherein said plastic material is a highlycrystalline polymer of a mono-l-olefin having 2 to 4 carbon atoms permolecule.

3. The film of claim 2 wherein said material contains from 10 to 15weight percent of an amorphous foreign polymer.

References Cited UNITED STATES PATENTS 2,307,056 1/ 1943 Minich 2642882,547,736 4/1951 Blake 264288 2,753,591 7/1956 Stevens et a1 18-13,038,198 :6/ 1962 Schaar 181 1,921,456 8/1933 Laney 161-128 2,279,3664/ 1942 Childs 16l-128 3,137,746 6/ 1964 Seymour et al.

ROBERT F. BURNETT, Primary Examiner.

L. M. CARLIN, Assistant Examiner.

1. A PRE-STRETCHED FILM OF ORIENTABLE PLASTIC MATERIAL HAVING REGULARLYSPACED ACROSS THE FACE THEREOF IN LONGITUDINAL STRIPS A PATTERN OF FINETRANSVERSE SHEARING LINES AND BETWEEN SAID STRIPS A CRISS-CROSS PATTERNOF SHEARING LINES AT ANGLES OF ABOUT 45* TO 60* TO SAID TRANSVERSELINES, SAID FILM BEING UNIFORMLY ORIENTABLE BY SIMPLE COLD STRETCHING INTHE LONGITUDINAL DIRECTION.